Image processing apparatus

ABSTRACT

There is provided a color region extending section that extends a color region with an image processing apparatus. Thereby, even if color pixels existing in a predetermined size of pixel matrix region decreases around a boundary of a color halftone dot region and a black character region and a count value of the color pixel approximates a reference value, the color region can be discriminated as so precisely. As a result, a color halftone dot region can be discriminated as so precisely.

This application is based on Application No. 2001-1225 filed in Japan,contents of which is hereby incorporated by reference. BACKGROUND OF THEINVENTION

[0001] 1. FIELD OF THE INVENTION

[0002] The present invention relates to an image processing apparatusthat discriminates attributes of each pixel and changes processingmethods in accordance with a discrimination result. More particularly,the present invention relates to an image processing apparatus capableof precisely discriminating a color halftone dot region from other typesof image regions.

[0003] 2. DESCRIPTION OF RELATED ART

[0004] In a digital copying machine or the like for reading an image byusing a CCD sensor and decomposing the read image into image pixels,thereby carrying out image processing, an optimal image processingmethod differs depending on a character image, a photography image, anda halftone dot image. Because of this, in such a digital copying machineor the like, the image attribute is discriminated, whereby the imageprocessing method is changed according to the discrimination result.Accordingly, precision is required when discriminating image attributes.Generally, an edge enhancement processing is applied to an imagediscriminated as a character image and a smoothing processing is appliedto an image discriminated as a photographic image or a halftone dotimage. Furthermore, It has been well-know that in case that an edgeenhancement processing is to be applied to a character region in ahalftone dot image, the edge enhancement processing is applied ifhalftone dots in the halftone dot image are color whereas a smoothingprocessing is applied if the halftone dots are black.

[0005] With the following reasons, an edge enhancement processing isapplied to a character region in a halftone dot image in case that thehalftone dots are color. That is, as shown in FIG. 24A, if blackhalftone dots are erroneously discriminated as a black character,proportional difference between black(K) and cyan(C), magenta(M),yellow(Y) becomes larger. Thereby, hue of a reproduction image isdifferent from that of an original image and the reproduction image islikely to have image noises. On the other hand, as shown in FIG. 24B,even if color halftone dots are erroneously discriminated as a colorcharacter, proportion difference between black(K) and cyan(c),magenta(M), yellow(Y) does not change. Therefore, hue of a reproductionimage does not differ compared with that of an original image. This isbecause image noises do not stand out in the reproduction image.Accordingly, discrimination of a character region in a black halftonedot region is not carried out but discrimination of a character regionin a color halftone dot region is carried out. Therefore, edgeenhancement processing is applied to a character region if the characterregion is in a color halftone dot region, thereby reproducing image withhigh quality.

[0006] However, in the above described conventional image processingapparatus, there has been a problem that if a color halftone dot imagecontains a black character in there, its color halftone dot region canhardly be discriminated as so precisely. That is, there has been a fearthat color halftone dot regions around boundaries of a black charactercan hardly be discriminated precisely. This is because the number ofcolor pixels existing in a predetermined region including a target pixeldecreases around the boundary of the color halftone dot region and theblack character region, whereby a count value of the color pixelsapproximates to a discrimination value. From this reason, there has beenthe fear that a color halftone dot region is erroneously discriminatedas a black halftone dot region. If the color halftone dot region iserroneously discriminated as a black halftone dot region, a smoothingprocessing is also applied to a character region in the color halftonedot region. Therefore, even if it is a character in a color halftone dotimage, an image of the character is not always reproduced precisely.

SUMMARY OF THE INVENTION

[0007] The present invention has been made in order to solve theforegoing problem. It is an object of the present invention to providean image processing apparatus for precisely discriminating a colorhalftone dot region regardless of color of a character in the colorhalftone dot image, thereby making it possible to reproduce thecharacter image in the color halftone dot image with high precision.

[0008] According to one aspect of the present invention, there isprovided an image processing apparatus, comprising: a firstdiscrimination unit that discriminates whether or not a target pixelbelongs to a halftone dot region; a second discrimination unit thatdiscriminates whether or not the target pixel belongs to a color region;a color region extending section that recognizes an extended region asan extended color region on condition that the extended region is formedby pixels that, the second discrimination unit has discriminated, belongto a color region; and a color halftone dot region discriminating unitthat discriminates a region as a color halftone dot region on conditionthat the region is recognized as an extended color region by the colorregion extending section and is formed by pixels that, the firstdiscrimination unit has discriminated, belong to a halftone dot region.

[0009] In this image processing apparatus, firstly, an attribute of atarget pixel is determined. That is, the first discrimination unitdiscriminates whether or not the target pixel belongs to a halftone dotregion. Furthermore, the second discrimination unit discriminateswhether or not the target pixel belongs to a color region. Next, thecolor region extending section recognizes an extended region as anextended color region on condition that the extended region is formed bypixels that, the second discrimination unit has discriminated, belong toa color region.

[0010] The color region extending section is provided and a color regionis extended so as to avoid erroneous discrimination of a color region.That is, the above items prevent a situation such that, in case that ablack character exists in a color region, the number of color pixelsexisting in a predetermined region including a target pixel decreasesaround the boundary of the color halftone dot region and the blackcharacter region and a count value of the color pixels approximates to adiscrimination value. Thereby, the color region is discriminatedprecisely even if a black character exists in a color region.

[0011] Subsequently, a region that meets the following two conditions isfinally discriminated as a color halftone dot region: (1) a region thatthe color halftone dot region discrimination unit has discriminated asan extended color region recognized by the color region extendingsection; and (2) a region formed by pixels that, the firstdiscrimination unit has discriminated, belong to a halftone dot region.Since a color region is thus precisely discriminated, a color halftonedot region is precisely discriminated at the color halftone dot regiondiscrimination unit. That is, even if a black character exists in acolor halftone dot region, the color halftone dot region is preciselydiscriminated as so at the color halftone dot region discriminationunit.

[0012] As has been described above, according to the image processingapparatus of the present invention, a color halftone dot region can beprecisely discriminated regardless of color of character in the colorhalftone dot region. Therefore, a character in a color halftone dotregion can be reproduced with high precision because an edge enhancementprocessing can surely be applied to a character region in the colorhalftone dot region.

[0013] According to another aspect of the present invention, there isprovided an image processing apparatus, comprising: a firstdiscrimination unit that discriminates whether or not each pixel belongsto a halftone dot region based on image data; a second discriminationunit that discriminates whether or not each pixel belongs to a colorregion based on image data; a color region extending section thatextends a color region formed by pixels that, the second discriminationunit has discriminated, belong to the color region; a color halftone dotregion discrimination unit that discriminates pixels that belong to acolor halftone dot region on condition that the pixels are included in acolor region extended by the color region extending section,discriminated by the first discrimination unit such that the pixelsbelong to a halftone dot region; and an image process unit that correctsimage data based on a discrimination result obtained by the colorhalftone dot region discrimination unit.

[0014] In this image processing apparatus as well, image processingsimilar to the afore-mentioned image processing apparatus is carriedout. That is, firstly, the first discrimination unit discriminates apixel that belongs to a halftone dot region and the seconddiscrimination unit discriminates a pixel that belongs to a colorregion. Next, the color region extending section extends a color regionformed by pixels that, the second discrimination unit has discriminated,belong to a color region. Then, the color halftone dot regiondiscriminating unit discriminates pixels that belong to a color halftonedot region on condition that the pixels are included in a color regionextended by the color region extending section, discriminated by thefirst discrimination unit such that the pixels belong to a halftone dotregion. Finally, the image process unit corrects image data based on adiscrimination result obtained by the color halftone dot regiondiscrimination unit. In the image process unit, edge enhancementprocessing is surely applied to the character region in the colorhalftone dot region so as to correct image data. Accordingly, acharacter in a color halftone dot region can be reproduced with highprecision

[0015] According to still another aspect of the present invention, thereis provided an image processing method comprising: a step 1 ofdiscriminating whether or not each pixel of image data belongs to ahalftone dot region as well as whether or not each pixel of image databelongs to a color region; a step 2 of extending a color region formedby pixels that, the step 1 has determined, belong to a color region; astep 3 of discriminating pixels that belong to a color halftone dotregion on condition that the pixels are included in a color regionextended by the step 2, and discriminated by the step 1 such that thepixels belong to a halftone dot region; and a step 4 of correcting imagedata based on a discrimination obtained by the step 3.

[0016] In this image processing method, firstly, the step 1discriminates whether or not each pixel of image data belongs to ahalftone dot region as well as whether or not each pixel of image databelong to a color region. Next, the step 2 extends a color region formedby pixels that, the step 1 has determined, belong to the color region.Then, the step 3 discriminates pixels that belong to a color halftonedot region on condition that the pixels are included in a color regionextended by the step 2, and discriminated by the step 1 such that thepixels belong to a halftone dot region. Finally, the step 4 correctsimage date based on a discrimination result obtained by the step 3.

[0017] As has been described above, according to the data processingmethod of the present invention, a pixel that meets the following twoconditions is discriminated as a pixel that belongs to a color halftonedot region: (1) the pixel is included in an extended color region; and(2) the pixel belongs to a halftone dot region. Then, image datacorrection is carried out based on the discrimination result. Therefore,even if the number of color pixels existing in a predetermined regionincluding a target pixel decreases around the boundary of the colorhalftone dot region and the black character region and a count value ofthe color pixels approximates to a discrimination value, the colorregion is discriminated as so precisely. Since a color region can bediscriminated precisely, a color halftone dot region can bediscriminated precisely, as well. Accordingly, with the inventive imageprocessing method, a color halftone dot region can be discriminated asso precisely regardless of color of a character in the color halftonedot image, thereby making it possible to reproduce the character imagein the color halftone dot image with high precision.

[0018] Additional objects and advantages of the invention will be setforth in the description which follows, and in part will be obvious fromthe description, or may be learned by practice of the invention. Theobjects and advantages of the invention may be realized and obtained bythe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF DRAWINGS

[0019] For a better understanding of the present invention, reference ismade to the following detailed description of the invention, just inconjunction with the accompanying drawings in which:

[0020]FIG. 1 is a block diagram depicting a schematic construction of acolor image processing apparatus according to one embodiment of thepresent invention;

[0021]FIG. 2 is a block diagram depicting a schematic construction of aregion discriminating portion shown in FIG. 1;

[0022]FIG. 3 is a block diagram depicting a schematic construction of acolor signal generating section in FIG. 2;

[0023]FIG. 4 is a block diagram depicting a schematic construction of avarious-edge signal generating sections shown in FIG. 2;

[0024]FIG. 5 is a view showing a primary differential filter (mainscanning direction);

[0025]FIG. 6 is a view showing a primary differential filter(sub-scanning direction);

[0026]FIG. 7 is a view showing a secondary differential filter (+ type);

[0027]FIG. 8 is a view showing a secondary differential filter (× type);

[0028]FIG. 9 is a view showing an external/internal edge discriminatingfilter;

[0029]FIG. 10 is a view showing an isolation point detection filter;

[0030]FIG. 11 is a view illustrating a discrimination method in theexternal/internal edge discriminating section in FIG. 4;

[0031]FIG. 12 is a block diagram depicting a schematic construction of ahalftone dot/color region signal generating section in FIG. 2;

[0032]FIG. 13 is a view illustrating the processing content at ahalftone dot region extension processing section shown in FIG. 12;

[0033]FIG. 14 is a block diagram depicting a schematic construction of ahalftone dot internal character region signal generating section shownin FIG. 2;

[0034]FIG. 15 is a view showing a part of the contents of a table at anMTF control signal generating section shown in FIG. 2;

[0035]FIG. 16 is a block diagram depicting a schematic construction ofan MTF correcting section shown in FIG. 1;

[0036]FIG. 17 is a view showing a −45 degree differential filter;

[0037]FIG. 18 is a view showing a 0 degree differential filter;

[0038]FIG. 19 is a view showing a +45 degree differential filter;

[0039]FIG. 20 is a view showing a +90 degree differential filter;

[0040]FIG. 21 is a view showing a smoothing filter;

[0041]FIG. 22 is a view showing a min filter;

[0042]FIG. 23 is a view specifically illustrating a method ofdiscriminating a character region in a halftone dot image;

[0043]FIG. 24 is a view illustrating why whole smoothing processing isapplied to a black halftone dot region; and

[0044]FIG. 25 is a view showing effects obtained by extending a colorregion.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0045] Reference will now be made in detail to the presently preferredembodiments of the invention as illustrated in the accompanyingdrawings, in which like reference numerals designate like orcorresponding parts.

[0046] Hereinafter, the best mode embodying an image processingapparatus according to the present invention will be described in detailwith reference to the accompanying drawings. According to the presentembodiment, the present invention is applied to a color image processingapparatus. In the present embodiment, a halftone dot internal characterdenotes a character image drawn in a halftone dot image expressed byhalftone dots, a background of which is expressed by halftone dots. Inaddition, a halftone dot internal character region denotes a region of acharacter image drawn in the halftone dot image.

[0047] A schematic construction of a color image processing apparatusaccording to the present embodiment is shown in FIG. 1. This color imageprocessing apparatus comprises: a CCD sensor 11; an image composingsection 12; an A/D converting section 13; a shading correcting section14; an interline correcting section 15 that carries out correctionbetween lines; a color aberration correcting section 16 that corrects acolor aberration of each color; a magnification change/movementprocessing section 17; a color converting section 18; a color correctingsection 19; a region discriminating section 20; an MTF correctingsection 21; and a printer interface (I/F) 22.

[0048] The CCD sensor 11 receives reflection light obtained by scanninga document with a scanner, converts the obtained light in aphotoelectric manner, and acquires an analog RGB signal. The imagecomposing section 12 composes an odd (odd numbered component) and even(even numbered component) with each other relevant to a respective oneof the analog RGB signals acquired by the CCD sensor 11. The A/Dconverting section 13 converts the analog RGB signal composed by theimage composing section 12 into a digital signal. The image composingsection 12 and A/D converting section 13 are provided for a respectiveone of the RGB signals.

[0049] The shading correcting section 14 eliminates non-uniformity oflight quantity in the main scanning direction on an image. Specifically,prior to a document reading operation, the reflection light from a whiteplate for shading correction is received by the CCD sensor 11, theobtained analog data is converted into digital data, and the digitaldata is stored in a memory. During document reading, the document readdata is corrected while the digital data stored in the memory is definedas a reference value.

[0050] The magnification change/movement processing section 17 controlsmemory write and readout operations, thereby carrying outenlargement/reduction processing in the main scanning direction of animage and image movement processing. The color converting section 18converts a current color into a specified display color system, whereinLab data is produced based on the RGB signal. Then, the Lab dataproduced at the color converting section 18 is inputted to the colorcorrecting section 19 and region discriminating section 20. The colorcorrecting section 19 produces a recording density signal CMYK which canbe recorded by a desired color considering spectroscopy characteristicsof an actually used four-color toner and a recording process based onthe Lab data.

[0051] The region discriminating section 20 discriminates an imageattribute for each pixel, generates signals that correspond to eachattribute so as to finally generate a control signal (CMPX, KMPX) for anMTF correcting section 21 based on these signals. This regiondiscriminating section 20 comprises a color signal generating section30, a various-edge signal generating sections 31, a halftone dot/colorregion signal generating section 32, a halftone dot internal characterregion signal generating section 33, and an MTF control signalgenerating section 34 as shown in FIG. 2.

[0052] The color signal generating section 30 generates a color signal(COLOR) and a black region signal (_BLACK) based on the Lab datagenerated at the color converting section 18. The color signalgenerating section 30 is composed of a converting section 35, a colordiscriminating threshold table 36, and a black discriminating thresholdtable 37, and two comparators 38 and 39, as shown in FIG. 3.

[0053] The converting section 35 generates saturation data (W7-0) inaccordance with a conversion formula ({square root}(a²+b²)) using data(a7-0, b7-0) generated at the color converting section 18. The colordiscrimination threshold table 36 generates a threshold for generating acolor signal (COLOR) based on the lightness data (L7-0). In addition,the black discrimination threshold table 37 generates a threshold forgenerating a black region signal (_BLACK) based on lightness data(L7-0). The thresholds for generating the color signal (COLOR) and blackregion signal (_BLACK) are generated based on the lightness data (L7-0)because the saturation quantity non-linearly depends on the lightness.

[0054] With such a construction, the color signal generating section 30compares the saturation data (W7-0) with the threshold generated by thecolor discrimination threshold table 36 in a comparator 38, therebygenerating a color signal (COLOR). In addition, in a comparator 39, thissection compares the saturation data (W7-0) with the threshold generatedby the black discrimination threshold table 37, thereby generating ablack region signal (_BLACK).

[0055] Turning to FIG. 2, the various-edge signal generating section 31generates a halftone dot discrimination isolation point signal (WAMI,KAMI), a halftone dot internal character region detection edge signal(_EDGL), a halftone dot internal character region detection internaledge signal (_INEDG), and a character edge region signal (_EDG) based onthe lightness data (L7-0). Then, in order to generate these signals, thevarious-edge signal generating section 31 includes: a matrix generatingsection 41; a characteristic quantity sampling filter section 42; twoselectors 43 and 44; four comparators 45 to 48; an external / internaledge discriminating section 49; two OR circuits 50 and 51, as shown inFIG. 4.

[0056] The matrix generating section 41 generates matrix data of 5×5pixel size from input image data. Then, filter processing caused by thecharacteristic quantity sampling filter 42 is applied to matrix datagenerated at the matrix generating section 41. The characteristicquantity sampling filter section 42 comprises: primary differentialfilters (main scanning direction and sub-scanning direction) 52 and 53;secondary differential filters (+ type and × type) 54 and 55; anexternal/internal edge discriminating filter 56; and an isolation pointdetecting filter 57. In the present embodiment, the primary differentialfilter 52 in the main scanning direction shown in FIG. 5 is used, andthe primary differential filter 53 in the sub-scanning direction shownin FIG. 6 is used. In addition, the + type secondary differential filter54 shown in FIG. 7 is used, and the × type secondary differential filter55 shown in FIG. 8 is used. Further, the external/internal edgediscriminating filter 56 shown in FIG. 9 is used.

[0057] Furthermore, the isolation point detecting filter 57 shown inFIG. 10 is used. This isolation point detecting filter 57 includes awhite isolation point detecting filter 57 a and a black isolation pointdetecting filter 57 b. In the case where the lightness value of a targetpixel V33 is greater than that of the peripheral eight pixels, and isgreater than the average lightness value of two pixels in eightdirections, the white isolation point detecting filter 57 a detects thatthe halftone dot discrimination isolation point signal (WAMI) is “H”active.

[0058] Namely, in the case where the lightness value of the target pixelV33 meets the following condition, it is discriminated that the targetpixel V33 is a white isolation point (WAMI=“H”). In this case, all ofthe conditions indicated below must be satisfied:

[0059] V33>MAX (V22, V23, V24, V32, V34, V42, V43, V44); and

[0060] V33>(V11+V22)/2+OFFSET; and

[0061] V33>(V13+V23)/2+OFFSET; and

[0062] V33>(V15+V24)/2+OFFSET; and

[0063] V33>(V31+V32)/2+OFFSET; and

[0064] V33>(V35+V34)/2+OFFSET; and

[0065] V33>(V51+V42)/2+OFFSET; and

[0066] V33>(V53+V43)/2+OFFSET; and

[0067] V33>(V55+V44)/2+OFFSET.

[0068] In addition, in the case where the lightness value of the targetpixel V33 is smaller than that of the periphery eight pixels, and issmaller than the average lightness value of two pixels in eightdirections, the black isolation point detecting filter 57 b detects thatthe halftone dot discrimination isolation point signal (KAMI) is “H”active.

[0069] Namely, in the case where the lightness value of the target pixelV33 meets the following condition, it is discriminated that the targetpixel V33 is a black isolation point (KAMI=“H”). In this case, all ofthe conditions indicated below must be satisfied:

[0070] V33<MIN (V22, V23, V24, V32, V34, V42, V43, V44); and

[0071] V33+OFFSET<(V11+V22)/2; and

[0072] V33+OFFSET<(V13+V23)/2; and

[0073] V33+OFFSET<(V15+V24)/2; and

[0074] V33+OFFSET<(V31+V32)/2; and

[0075] V33+OFFSET<(V35+V34)/2; and

[0076] V33+OFFSET<(V51+V42)/2; and

[0077] V33+OFFSET<(V53+V43)/2; and

[0078] V33+OFFSET<(V55+V44)/2.

[0079] OFFSET is a threshold for discriminating an isolation point.

[0080] Turning to FIG. 4, an output from the primary differential filter52 in the main scanning direction is inputted to terminal A of theselector 43, and an output from the primary differential filter 53 inthe sub-scanning direction is outputted to terminal B of the selector43. In addition, an output from the + type secondary differential filter54 is inputted to terminal A of the selector 44, and an output from the× type secondary differential filter 55 is inputted to terminal B of theselector 44. In the selectors 43 and 44 each, a greater one of thevalues inputted to terminals A and B is selected and outputted.

[0081] In addition, an output (EDG07-00) from the selector 43 isinputted to terminal P of the comparator 45, and an edge reference value(EDGREF07-00) is inputted to terminal Q of the comparator 45. Similarly,an output (EDG07-00) from the selector 43 is inputted to terminal P ofthe comparator 46, an edge reference value (EDGREF17-10) is inputted toterminal Q of the comparator 46. Cn the other hand, an output (EDG17-10)from the selector 44 is inputted to terminal P of the comparator 47, andan edge reference value (EDGREF27-20) is inputted to terminal Q of thecomparator 47. Similarly, an output (EDG17-10) from the selector 44 isinputted to terminal P of the comparator 48, and an edge reference value(EDGREF37-30) is inputted to terminal Q of the comparator 48.

[0082] An output of the comparator 45 and that of the comparator 47 areinputted to an OR circuit 50. In addition, an output of the comparator46 and that of the comparator 48 are inputted to an OR circuit 51. Withthe above construction, in the OR circuit 50, in the case where any ofthe following conditions (1) and (2) is met, a character edge regionsignal (_EDG) is set to “L” active. The conditions include: (1) a casein which the maximum value of values filtered by the primary filter 52in the main scanning direction and the primary filter 53 in thesub-scanning direction is greater than an edge reference value(EDGREF07-00); and (2) a case in which the maximum value of valuesfiltered by the + type secondary differential filter 54 and × typesecondary differential filter 55 is greater than an edge reference value(EDGREF27-20).

[0083] Similarly, in the OR circuit 51, in the case where any of thefollowing conditions (3) and (4) is met, a halftone dot internalcharacter region detection edge signal (_EDGL) is set to “L” active. Theconditions include: (3) a case in which the maximum value of valuesfiltered by the primary filter 52 in the main scanning direction and theprimary filter 53 in the sub-scanning direction is greater than an edgereference value (EDGREF17-10); and (4) a case in which the maximum valueof values filtered by the + type secondary differential filter 54 and ×type secondary differential filter 55 is greater than an edge referencevalue (EDGREF37-30).

[0084] The value filtered by the external/internal edge discriminatingfilter 56 and the discrimination reference value (INOUT7-0) are inputtedto the external/internal edge discriminating section 49. In theexternal/internal edge discriminating section 49, the external/internaledge is discriminated as shown in FIG. 11. That is, when INOUT7=0 issatisfied, where the edge detection quantity is set to a positive value(FL238=0) and is greater than a threshold (INOUT6-0), it isdiscriminated as an internal edge. In addition, in the case where theedge detection quantity is set to a positive value (FL238=1) and issmaller than the threshold (INOUT6-0), or alternatively, where the edgedetection quantity is set to a negative value (FL238=1), it isdiscriminated as an external edge. On the other hand, when INOUT7=1 issatisfied, where the edge detection quantity is set to a positive value(FL238=0), or alternatively, where the edge detection quantity is set toa negative value (FL238=1) and is smaller than the threshold (INOUT6-0),it is discriminated as an internal edge. In addition, in the case wherethe edge detection quantity is set to a negative value (FL238=1) and isgreater than the threshold (INOUT6-0), it is discriminated as anexternal edge. Where the external/internal edge discriminating section49 discriminates a target to be an internal edge, the halftone dotinternal character region detection internal edge signal (_INEDG) is setto “L” active. The threshold (INOUT6-0) and edge detection quantity(FL237-230) indicates an absolute value.

[0085] Turning to FIG. 2 again, the halftone dot/color region signalgenerating section 32 generates a color region signal (_COL_DOT) and ahalftone dot region signal (_AMI) based on the color signal (COLOR) andhalftone dot discrimination isolation point signal (WAMI, KAMI). Thatis, the halftone dot/color region signal generating section 32discriminates whether it is halftone dot region or a color region. Inthe case that the halftone dot/color region signal generating section 32activates both a color region signal (_COL_DOT) and a halftone dotregion signal (_WAMI) to a pixel, it is discriminated that the pixelbelongs to color halftone dot region in the region discriminatingsection 20. This halftone dot/color region signal generating section 32includes: a black isolation point counting section 60; a white isolationpoint counting section 61; a color pixel counting section 62; an adder63; four comparators 64 to 67; an OR circuit 68; a halftone dot regionextension processing section 69; and a color region extension processingsection 70, as shown in FIG. 12.

[0086] The black isolation point counting section 60 counts the numberof black isolation points that exist in a 9×45 pixel matrix region.Similarly, the white isolation point counting section 61 counts thenumber of white isolation points that exist in a 9×45 pixel matrixregion. Outputs from the black isolation point counting section 60 areinputted to terminal A of the adder 63 and terminal P of the comparator65, respectively. On the other hand, outputs from the white isolationpoint counting section 61 are inputted to terminal B of the adder 63 andterminal P of the comparator 66, respectively. In addition, an outputfrom the adder 63 is inputted to terminal P of the comparator 64.Reference values (CNTREF17-10, 27-20, and 37-30) are inputted toterminals P of comparators 64 to 66. In addition, outputs from thecomparators 64 to 66 are inputted to an OR circuit 68.

[0087] In an OR circuit 68, if there is met at least one of theconditions wherein a total number of black and white isolation points isgreater than the reference value (CNTREF17-10); the number of blackisolation points is greater than the reference value (CNTREF27-20); andthe number of white isolation points is greater than the reference value(CNTREF37-30), it is discriminated that the target pixel belongs to ahalftone dot region. Then, the halftone dot region signal (_AMI) is setto “L” active. Then, region expansion processing caused by a halftonedot region extension processing section 69 is applied to the halftonedot region signal (_AMI).

[0088] In the halftone dot region extension processing section 69, asshown in FIG. 13, if any of the pixels (AMI1 to AMI9) corresponding to aposition distant by 16 halftone dots in the main scanning direction andby four lines in the sub-scamming direction relevant to the target pixel(AMI5), AMI5 is defined as a halftone dot point irrespective of whetheror not AMI5 is a halftone dot, thereby carrying out extension processingof the halftone dot region. Specifically extension processing is carriedout in accordance with the formula below:

!AMI=!AMI1#!AMI2#!AMI3#!AMI4 #!AMI5#!AMI6#!AMI7#!AMI8#!AMI9

[0089] wherein “!” denotes inversion processing, and “#” denotes ORprocessing, respectively.

[0090] A halftone dot region is thus extended at the halftone dot regionextension processing section 69, whereby a situation such as below canbe avoided: isolation points existing in a 9×45 pixel matrix decreasesaround a boundary of a halftone dot region and a solid color region, anda count value of the isolation points approximates to a reference value.Since the situation such as the above can be avoided, the halftonedot/color region signal generating section 32 can discriminate ahalftone dot region more precisely.

[0091] In addition, the color pixel counting section 62 counts thenumber of color pixels that exist in a 9×45 pixel matrix region. Anoutput from the color pixel counting section 62 is inputted to terminalP of the comparator 67. A reference value (CNTREF47-40) is inputted toterminal Q of the comparator 67. In this manner, in the case where thenumber of color pixels is greater than the reference value(CNTREF47-40), the comparator 67 discriminates that the target pixel isa color pixel, and a color region signal (_COL_DOT) is set to “L”active. Then, region extension processing caused by a color regionextension processing section 70 is applied to the color region signal(_COL_DOT).

[0092] The region extension processing at the color region extensionprocessing section 70 is carried out in the same manner as that at ahalftone dot region extension processing section 69. That is, if any oneof the pixels that are sixteen-line away from a main scanning directionand four-line away from a sub-scanning direction with reference to thetarget pixel is a color pixel, the target pixel is regarded as a colorpixel regardless of color of the target pixel, and a color regionextension processing is carried out.

[0093] Since the color region extension processing section 70 thusextends a color region, the color region can be discriminated as soprecisely even though color pixels existing in a 9×45 pixel matrixaround a boundary of a color region and a black character regiondecreases and a count number of the color pixels approximates to areference value, a color region can be discriminated as so precisely.

[0094] Since a halftone dot region signal (_AMI) and a color regionsignal (_COL_DOT) are generated by the halftone dot/color region signalgenerating section 32 precisely, the region discriminating section 20can discriminate a color halftone dot region precisely.

[0095] Turning to FIG. 2, the halftone dot internal character regionsignal generating section 33 generates a halftone dot internal characterregion signal (_MOJI) based on the halftone dot discrimination isolationsignals (WAKI, KAMI), a halftone dot internal character region detectionedge signal (_EDGL), and a dot internal character region detectioninternal edge signal (_INEDG). That is, the halftone dot internalcharacter region signal generating section 33 discriminates a halftonedot internal character region from other types of character regions.This halftone dot internal character region signal generating section 33includes: an OR circuit 75; two AND circuits 78 and 82; an isolationpoint counting section 76; an internal edge counting section 79; twocomparators 77 and 88; and a continuity detecting section 81, as shownin FIG. 14.

[0096] The isolation point counting section 76 counts the number ofisolation points that exist in an 11×11 pixel matrix region. A signalfrom the OR circuit 75, i.e., a signal indicative of whether or not anisolation point exists relevant to each pixel, is inputted to theisolation point counting section 76. This signal is held by an 11×11pixel matrix whose center is the target pixel, and the pixelscorresponding to the isolation point are counted. In addition, theinternal edge counting section 79 counts the number of internal edgepixels that exists in a 3×3 pixel matrix region. A signal from the ANDcircuit 78, i.e., a signal indicative of whether or not a pixel belongsto an internal edge region that is a part of an edge region, is inputtedto the internal edge counting section 79 with respect to each pixel.This signal is held by a 3×3 pixel matrix whose center is the targetpixel, and the pixels belonging to the internal edge region that is apart of edge region are counted. Further, the continuity detectingsection 81 detects the continuity of the internal edge, namely, whetheror not the pixels belonging to the internal edge region continuouslyexist.

[0097] The halftone dot discrimination isolation point signals (WAMI,KAMI) are input to the OR circuit 75. Thus, the OR circuit 75 computes alogical sum between WAMI and KAMI. The computation result is inputted tothe isolation point counting section 76. Further, an output from theisolation point counting section 76 is inputted to terminal P of thecomparator 77. On the other hand, a reference value (CNTREF57-50) isinputted to terminal Q of the comparator 77.

[0098] In addition, a halftone dot internal character region detectionedge signal (_EDGL) and a halftone dot internal character regiondetection internal edge signal (_INEDG) are inputted to the AND circuit78. The AND circuit 78 computes a logical product between _EDGL and_INEDG. Then, the computation result is inputted to the internal edgecounting section 79. Further, an output from the internal edge countingsection 79 is inputted to terminal P of the comparator 80. On the otherhand, a reference value (CNTREF67-60) is inputted to terminal Q of thecomparator 80.

[0099] An output from the comparator 80 is inputted to the continuitydetecting section 81. At this continuity detecting section 81, withrespect to matrix data 83 of 5×5 pixel size in which a target pixel a33is positioned at its center, it is detected whether or not threecontinuous pixels whose logic is the same as that of the target pixela33 exist in any of the shown eight directions, whereby the continuityof the internal edges is detected. Specifically, the continuity of theinternal edges is detected in accordance with the formula below.

!Y=(a11×a22×a33)+(a13×a23×a33) +(a15×a24a33)+(a35×a34×a33)+(a55×a44×a33)+(a53×a43×a33) +(a51×a42×a33)+(a31×a32×a33)

[0100] where “!” denotes inversion processing, “×” denotes ANDprocessing, and “+” denotes OR processing, respectively.

[0101] In this way, at the continuity detecting section 81, thecontinuity of the internal edges is detected, and the detection resultis taken into consideration, thereby making it possible to prevent imagenoise or the like from being incorrectly discriminated as a characterregion in a halftone dot. That is, the character region in the halftonedot can be discriminated more precisely.

[0102] Finally, an output from the comparator 77 and an output from thecontinuity detecting section 81 are inputted to the AND circuit 82, andan output from the AND circuit 82 is obtained as an output of thehalftone dot internal character region signal generating section 33.With such a construction, at the halftone dot internal character regionsignal generating section 33, in the case where it is judged that thecount value of isolation points is smaller than a reference value(CNTREF57-50), the count value of internal edges is greater than areference value (CNTREF67-60), and further, the internal edges arecontinuous, the target pixel is judged as belonging to the halftone dotinternal character region. At the halftone dot internal character regionsignal generating section 33, in the case where it is judged that thetarget pixel belongs to the halftone dot internal character region, thehalftone dot internal character region signal (_MOJI) is set to “L”active.

[0103] Turning to FIG. 2 again, the MTF control signal generatingsection 34 generates an MTF control signal (CMPX2-1, KMPX2-0) thatcontrols an operation of the MTF control section 21 based on a colorregion signal _COL_DOT), a halftone dot region signal (_AMI), a halftonedot internal character region signal (_MOJI), and a halftone dotinternal character region detection internal edge signal (_INEDG),character edge region signal(_EDG), and black region signal (_BLACK) .This MTF control signal generating section 34 is composed of a lookuptable so as to generate an MTF control signal (CMPX, KMPX) while sixregion discrimination attribute signals (_COL_DOT, _AMI, _MOJI, _INEDG,_EDG, _BLACK) are defined as input addresses.

[0104]FIG. 15 shows a part of a table that configures the MTF controlsignal generating section 34. The MTF control signal (CMPX1-0, KMPX1-0)is a signal that indicates the content of base processing executed atthe MFT correcting section 34. In addition, an MTF control signal(CMPX2, KMPX2) is a signal that indicates whether or not edgeenhancement processing is carried out at the MTF correcting section 21.The MTF control signal (CMPX) is a signal for CMY, and the MTF controlsignal (KMPX) is a signal for K. At the MTF correcting section 21, inthe case where the MTF control signal (CMPX1-0, KMPX1-0) is 0, smoothingprocessing is executed; in the case where the signal is 1, minprocessing (minimization processing) is executed; and in the case wherethe signal is 2, no processing is carried out (hereinafter, referred toas “through processing”). In addition, in the case where the MTF controlsignal (CMPX2, KMPX2) is 0, edge enhancement processing is executed, andin the case where the signal is 1, edge enhancement processing is notexecuted.

[0105] Turning to FIG. 1, the MTF correcting section 21 corrects imagesharpness or the like. This MTF correcting section 21 comprises: a cyan(C) correcting section 90 corresponding to each color of CMYK; a magenta(M) correcting section 91; a yellow (Y) correcting section 92; and ablack (K) correcting section 93 as shown in FIG. 16 so as to carry outcorrection processing for four colors at the same time. The CMYcorrection processing is controlled by an MTF control signal (CMPX2-0)generated at the region discriminating section 20, and K correctionprocessing is controlled by the MTF control signal (KMPX2-0).

[0106] Now, a construction of each of the color correcting sections willbe described in more detail. There are provided with the cyan (C)correcting section 90, magenta (M) correcting section 91, yellow (Y)correcting section 92, and black (K) correcting section 93, each ofwhich has the same construction. Therefore, a construction of the cyan(C) correcting section 90 is described here, and descriptions of theother correcting sections are omitted here. This cyan (C) correctingsection 90 includes: a matrix generating section 100; a characterprocessing filter section 101 having a variety of filters; two selectors102 and 103; and an adder 104, as shown in FIG. 16.

[0107] The matrix generating section 100 generates 5×5 pixel matrixdata, and supplies the data to the character processing filter section101. The character processing filter section 101 is composed of: an edgeenhancement quantity generating filter section 110; a smoothing filtersection 111; a min filter section 112. With such a construction, thecharacter processing filter section 101 outputs edge enhancementquantity data, smoothing processing data, and min processing data.

[0108] The edge enhancement quantity generating filter section 110includes: a −45 degree differential filter section 120; a 0 degreedifferential filter section 121; a 45 degree differential filter section122; a 90 degree differential filter section 123; and a selector 124that selects a maximum value of the data filtered by these filtersections. In the present embodiment, the −45 degree differential filtersection 120 comprises a filter shown in FIG. 17; the 0 degreedifferential filter section 121 comprises a filter shown in FIG. 18; the45 degree differential filter 122 comprises a filter shown in FIG. 19;and the 90 degree differential filter section 123 comprises a filtershown in FIG. 20.

[0109] In addition, the smoothing filter section 111 comprises a filtershown in FIG. 21 so that smoothing processing is executed. Further, atthe min filter section 112, as shown in FIG. 22, processing is executedsuch that a minimum value of the 5×5 pixel matrix data is defined astarget pixel data.

[0110] An output from the edge enhancement quantity generating filtersection 110 is inputted to terminal A of the selector 102. “00” isinputted to terminal B of the selector 102, and an MTF control signal(CMPX2) is inputted to terminal S. In this manner, at the selector 102,either of the values inputted to terminals A and B is selected andoutputted depending on the contents of MTF control signal (CMP2).

[0111] That is, when the MTF control signal (CMPX2)=0, edge enhancementprocessing is executed, and thus, the value inputted to terminal A isselected and outputted at the selector 102. On the other hand, when theMTF control signal (CMPX2)=1, edge enhancement processing is notexecuted, and thus, the value inputted to terminal B is selected andoutputted at the selector 102.

[0112] In addition, an output from the smoothing filter section 111 isinputted to terminal A of the selector 103, and an output from the minfilter section 112 is inputted to terminal B of the selector 103.Further, outputs from the matrix generating section 100, i.e., dataobtained when no processing is done by the character processing filtersection 101 are inputted to terminals C and D of the selector 103.Furthermore, an MTF control signal (CMPX1-0) is inputted to terminal Sof the selector 103. At the selector 103, any of the values inputted toterminals A to D is selected and outputted in accordance with thecontents of the MTF control signal (CMPX1-0).

[0113] That is, when the MTF control signal (CMPX1-0)=0, smoothingprocessing is required. Thus, at the selector 103, the value inputted toterminal A is selected and outputted. In addition, when the MTF controlsignal (CMPX1-0)=1, min processing is carried out. Thus, at the selector103, the value inputted to terminal A is selected and outputted.Further, when the MTF control signal (CMPX1-0)=2, through processing iscarried out. Thus, at the selector 103, the value inputted to terminal C(or D) is selected and outputted.

[0114] An output from the selector 102 and an output from the selector103 are inputted to terminals A and B of the adder 104 respectively. Inthis manner, at the adder 104, edge enhancement data (“00” if no edgeenhancement is carried out) and processing data selected by the selector103 are added, and the added data is obtained as an output (C7-0) fromthe MTF correcting section 21. Processing for the recording densitysignals (M, Y, K) of the other colors as well is executed in the samemanner as for cyan(C).

[0115] In this way, image data (C7-0, M7-0, Y7-0, K7-0) of each color inwhich processing is applied in the MTF correcting section 21 istransmitted to an image output device such as printer via a printerinterface (I/F) 22. Hence, reproduction image is obtained in the imageoutput device.

[0116] Now, an operation of the entire color image processing apparatushaving the above described construction will be briefly described here.First, document image information is read by the CCD sensor 11. Theanalog image data read by the CCD sensor 11 is converted into digitalimage data. Then, shading correction, interline correction, coloraberration correction, magnification change/movement processing, colorconversion processing, color correction, region discriminationprocessing, and MTF correction are applied sequentially to the digitalimage data. Then, based on the image data to which a variety of imageprocessing is applied, the document reproduction image is outputted on arecording medium by a printer or the like via the printer interface(I/F) 22.

[0117] The region discriminating section 20 discriminates where thetarget pixel belongs to among from a color region, a monochrome region,a halftone dot region, a character region, and a halftone dot internalcharacter region. With respect to the halftone dot region, characterregion, and halftone dot internal character region, it is discriminatedwhether or not these regions are based on color or monochrome.Discrimination of the color halftone dot region is carried out based ona discrimination result obtained by the halftone dot/color region signalgenerating section 32. That is, it depends on whether or not it is ahalftone dot region and whether or not it is a color region. Firstly, anmethod of halftone dot region discrimination will be described in thenext paragraph.

[0118] For discriminating halftone dot region, firstly, the blackisolation point counting section 60 and the white isolation pointcounting section 61 count the number of black isolation points and thatof white isolation points existing in a 9×45 pixel matrix, respectively,which is based on halftone dot discrimination isolation point signals(WAMI and KAMI) generated by the various edge signal generating section31. Next, the adder 63 obtains a sum of a count value of the blackisolation points and that of the white isolation points. Then, thecomparators 64 through 66 compare the sum of the black isolation pointsand the white isolation points, the count value of the black isolationpoints, and that of the white isolation points with their respectivereference values (CNTREF17-10, 27-20, 37-30). Comparison resultsobtained by the comparators 64 through 66 are inputted to the OR circuit68.

[0119] At least one of the following three conditions is met at the ORcircuit 68, it is discriminated that the target pixel belongs to ahalftone dot region. The conditions to be met are: (1) the sum of theblack isolation points and the white isolation points is larger than thereference value (CNTREF17-10); (2) the count value of the blackisolation points is larger than the reference value (CNTREF27-20); and(3) the count value of the white isolation points is larger than thereference value (CNTREF37-30). When at least one of the above conditionsis met, a halftone dot region signal (_AMI) is set to “L” active. Afterthat, the halftone dot region extension processing section 69 applies aregion extension processing to the halftone dot region signal (_AMI).

[0120] Next, a method of color region discrimination will be described.Firstly, the color pixel counting section 62 counts the number of colorpixels existing in a 9×45 pixel matrix, which is based on a color signal(COLOR) generated by the various edge signal generating section 31.Next, the comparator 67 compares the count value of the color pixelswith a reference value (CNTREF 47-40). If the count value is larger thanthe reference value (CNTREF 47-40), it is discriminated that the targetpixel belongs to a color region. Subsequently, a color region signal(_COL_DOT) is set to “L” active. After that, the color region extensionprocessing section applies a region extension processing to the colorregion signal (_COL_DOT).

[0121] In case a halftone dot region signal (_AMI) and a color regionsignal (_COL_DOT) are set active in accordance with ahalftone-dot-region discrimination and color-region discriminationresult obtained by the halftone dot/color region signal generatingsection 32, the region discriminating section 20 discriminates that thetarget pixel belongs to a color halftone dot region. Since regionextension processing is applied to the halftone dot region signal (_AMI)and the color region signal (_COL_DOT), halftone dot regiondiscrimination and color region discriminations are carried out moreprecisely. Accordingly, the region discriminating section 20 candiscriminate a color halftone dot region precisely.

[0122] Next, a method of discriminating a halftone dot internalcharacter region will be described. The halftone dot internal characterregion is discriminated at the halftone dot internal character regionsignal generating section 33 provided at the region discriminatingsection 20. The OR circuit 75 computes a logical sum of the halftone dotdiscrimination isolation point signal (WAMI, KAMI) generated by thevarious-edge signal generating section 31. Then, the logical sumcomputed by the OR circuit 75 is inputted to the isolation pointcounting section 76. Then, the isolation point counting section 76counts the number of white and black isolation points that exist in an11×11 pixel matrix region. Then, the comparator 77 compares theisolation point count value with the reference value (CNTREF57-50). Thecomparison result is inputted to the AND circuit 82.

[0123] On the other hand, in parallel to the above processing, the ANDcircuit 78 computes a logical sum between the halftone dot internalcharacter region detection internal edge signal (_EDGL) and halftone dotinternal character region detection edge signal (_INEDG) generated bythe various-edge signal generating section 31. Then, the logical productcomputed by the AND circuit 78 is inputted to the internal edge countingsection 79. Then, the internal edge counting section 79 counts thenumber of internal edges that exist in a 3×3 pixel matrix region. Then,the comparator 80 compares the internal edge count value with thereference value (CNTREF67-60). The comparison result is inputted to thecontinuity detecting section 81. Then, the continuity detecting section81 detects the continuity of the internal edges. Then, data concerningthe count value of the internal edges and the presence or absence ofcontinuity is inputted to the AND circuit 82.

[0124] Finally, at the AND circuit 82, in the case where the isolationpoint count value (WAMI+KAMI) is smaller than the reference value(CNTREF57-50), and moreover, the internal edge count value(_EDGL×_INEDG) is greater than the reference value (CNTREF67-60) and theinternal edges are continuous, the target pixel is judged as belongingto the halftone dot internal character region. Then, the halftone dotinternal character region signal (_MOJI) is set to “L” active. Thehalftone dot internal character region is discriminated for all thepixels of the input image data by repeatedly moving the target pixels inthe main scanning direction one by one and moving them by one pixel inthe subsidiary scamming direction when the pixel reaches a finalposition in the main scanning direction.

[0125] Now, discrimination processing at the halftone dot internalcharacter region signal generating section 33 will be described by wayof showing a specific example shown in FIG. 23. Here, a description willbe given by exemplifying discrimination processing in part of an image(region R in (A) of FIG. 23 (refer to (B) of FIG. 23) on which uppercaseletter H is drawn in the halftone dot as shown in (A) of FIG. 23.

[0126] First, in region R1 shown in (B) of FIG. 23, as shown in (C) ofFIG. 23, isolation points exist all over the region, and no internaledge exist. Thus, at the halftone dot internal character region signalgenerating section 33, the comparator 77 judges that the isolation pointcount value (WAMI+KAMI) is greater than the reference value(CNTREF57-50). In addition, the comparator 80 judges that the internaledge count value (_EDGL×_INEDG) is smaller than the reference value(CNTREF67-60). Further, the continuity detecting section 81 judges thatthe internal edge is not continuous. Therefore, the halftone dotinternal character region signal (_MOJI) is set to “H” by the ANDcircuit 82. That is, it is discriminated that the target pixel shown in(C) of FIG. 23 fails to belong to the halftone dot internal characterregion.

[0127] On the other hand, in region R2 shown in (B) of FIG. 23, as shownin (D) of FIG. 23, isolation points exist in half or less of the region,and the internal edges continuous at the center of the region exist.Thus, in the halftone dot internal character region signal generatingsection 33, the comparator 77 first judges that the isolation pointcount value (WAMI+KAMI) is smaller than the reference value(CNTREF57-50). In addition, the comparator 80 judges that the internaledge count value EDGL×INEDG) is greater than the reference value(CNTREF67-60). Further, the continuity detecting section 81 judges thatthe internal edges are continuous. Therefore, the halftone dot internalcharacter region signal (_MOJI) is set to “L” by the AND circuit 82.That is, the target pixel shown in(D) of FIG. 23 is discriminated asbelonging to the halftone dot internal character region.

[0128] As has been described above, the halftone dot internal characterregion can be discriminated because the halftone dot region and halftonedot internal character region are different from each other in theircharacteristics. That is, the halftone dot region is characterized inthat a large number of isolation points exist, a small number ofinternal edges exist, and the internal edges are not continuous. On theother hand, the halftone dot internal character region is characterizedin that a small number of isolation points exist, a large number ofedges exist, and the internal edges are continuous. Thus, the halftonedot region and halftone dot internal character region are completelyreversed in their characteristics. Therefore, the halftone dot internalcharacter region can be precisely discriminated by making best use of adifference in characteristics of these regions each.

[0129] As a result of region discrimination carried out at the regiondiscriminating section 20, i.e., in accordance with a variety of regionattribute discrimination signals, the MTF correcting section 21 appliesimage processing according to a variety of attributes. Specifically,smoothing processing is applied to the halftone dot region, and edgeenhancement processing is applied to the character region. Thus, when acharacter exists on a halftone dot image, smoothing processing isapplied to the halftone dot region excluding the character region, andedge enhancement processing is applied to the halftone dot internalcharacter region. However, it should be noted that smoothing processingis applied to an entirety of the black halftone dot region regardless ofpresence/absence of a character.

[0130] Smoothing processing is applied to an entirety of the blackhalftone dot region because of the following reason. As shown in FIG.24A, if a group of black halftone dots is erroneously discriminated as ablack character, proportion difference between black(K) and cyan(C),magenta(M), yellow(Y) becomes larger. Thereby, hue of a reproductionimage is different from that of an original image and the reproductionimage is likely to have image noises. Therefore, smoothing processing isapplied to an entirety of the black halftone dot region withoutcharacter region discrimination in a black halftone dot region so as toavoid image noises. On the other hand, as shown in FIG. 24B, even if agroup of color halftone dots is erroneously discriminated as a colorcharacter, proportion difference between black(K) and cyan(C),magenta(M), yellow(Y) does not change. Therefore, hue of a reproductionimage does not differ compared with that of an original image. From thisreason, smoothing processing is not necessary for an entirety of thecolor halftone dot region, different from case of a black halftone dotregion.

[0131] However, as shown in FIG. 25, in case a black character exists ina color halftone dot region and a color region is not extended, halftonedots around boundary of a character region discriminated as blackhalftone dots. That is, the color region cannot be discriminated as soprecisely. Once the color halftone dots around the boundary of thecharacter region is discriminated as black halftone dots, smoothingprocessing is applied to the character region. As a result, thecharacter is blurred. Therefore, a color image processing apparatus ofthe present invention has the color region extending section 70 toextend a color region. Thereby, color halftone dots around boundary of ablack character region is surely discriminated as so and edgeenhancement processing is surely applied to the black character in thecolor halftone dot region. Accordingly, a character in a color halftonedot region can be reproduced precisely without getting blurred.

[0132] As described in the above, the color image processing apparatusdirected to this embodiment has the color region extending section 70 toextend a color region. Thereby, even if color pixels existing in a 9×45pixel matrix region decreases around a boundary of a color halftone dotregion and a black character region and a count value of the color pixelapproximates a reference value, the color region can be discriminated asso precisely. Even if a black character exists in a color halftone dotregion, the halftone dot/color region signal generating section 32 candiscriminate the color region precisely. Accordingly, the color halftonedot region can be discriminated as so precisely. Furthermore, thehalftone dot internal character region signal generating section 33discriminates a halftone dot internal character region precisely.Therefore, edge enhancement processing can surely be applied to thecharacter region in the color halftone dot region. As a result, thecharacter in the color halftone dot region is reproduced with highquality.

[0133] The above described embodiment is provided for mere illustrativepurpose, and the present invention is not limited thereto. Of course,various modifications or variations can occur without departing thespirit of the invention. For example, in the above described embodiment,although an isolation point is detected as a halftone dotcharacteristic, any other point may be detected as far as such pointcharacterizes a halftone dot without being limited thereto. In addition,the present invention is applicable to an image processing apparatussuch as digital copying machine, printer, and facsimile machine otherthan above described embodiment. Further, specific numeral values shownin the above-described embodiment (for example, matrix size or the like)are provided for mere illustrative purposes.

What is claimed is:
 1. An image processing apparatus comprising: a firstdiscrimination unit that discriminates whether or not a target pixelbelongs to a halftone dot region; a second discrimination unit thatdiscriminates whether or not the target pixel belongs to a color region;a color region extending section that recognizes an extended region asan extended color region on condition that the extended region is formedby pixels that, the second discrimination unit has discriminated, belongto a color region; and a color halftone dot region discrimination unitthat discriminates a region as a color halftone dot region in case theregion is recognized as an extended color region by the color regionextending section and is formed by pixels that, the first discriminationunit has discriminated, belong to a halftone dot region.
 2. An imageprocessing apparatus according to claim 1, wherein the seconddiscrimination unit contains: a color pixel sampling section thatsamples color pixels from image data; and a first counter that countsthe number of color pixels in a first region, including a target pixel,among from the color pixels sampled by the color pixel sampling section,and wherein in case a count value obtained by the first counter exceedsa predetermined value, it is discriminated that the target pixel belongsto a color region.
 3. An image processing apparatus according to claim2, wherein the color pixel sampling unit generates lightness data andsaturation data of each pixel from image data, and samples color pixelsby comparing a threshold determined based on the lightness datagenerated with the saturation data generated.
 4. An image processingapparatus according to claim 1, wherein, in case at least a pixel that,the second discrimination unit discriminates, belongs to a color regionexists in a second region where a target pixel is centered, the colorregion extending section regards that the target pixel belongs to acolor region regardless of a discrimination result obtained by thesecond discrimination unit and determines that the color region is to beextended.
 5. An image processing apparatus according to claim 1, furtherincluding a halftone dot region extending section that recognizes aregion as an extended halftone dot region in case the region is anextended region including a pixel that, the first discrimination unitdiscriminates, belongs to a halftone dot region, wherein the colorhalftone dot region discrimination unit discriminates a region as acolor halftone dot region in case the region is recognized as anextended color region by the color region extending section andrecognized as an extended halftone dot region by the halftone dot regionextending section.
 6. An image processing apparatus according to claim1, wherein the first discrimination unit includes: ahalftone-dot-characteristic-point pixel sampling unit that samples ahalftone-dot-characteristic-point pixel indicative of halftone dotcharacteristics from image data; and a second counter that counts thenumber of pixels that exist in a third region, where a target pixel iscentered, from among halftone-dot-characteristic-point pixels sampled bythe halftone-dot-characteristic-point pixel sampling unit, wherein incase a count value of the second counter exceeds a predetermined value,it is discriminated that the target pixel belongs to a halftone dotregion.
 7. An image processing apparatus according to claim 6, whereinthe halftone-dot-characteristic-point pixel sampling unit contains afilter that detects an isolation points as a halftone dot characteristicpoint.
 8. An image processing apparatus according to claim 1 furtherincluding a correction unit that corrects image data based on adiscrimination result of the color halftone dot region discriminationunit.
 9. An image processing apparatus comprising: a firstdiscrimination unit that discriminates whether or not each pixel belongsto a halftone dot region based on image data; a second discriminationunit that discriminates whether or not each pixel belongs to a colorregion based on image data; a color region extending section thatextends a color region formed by pixels that, the second discriminationunit has discriminated, belong to the color region; a color halftone dotregion discrimination unit that discriminates pixels that belong to acolor halftone dot region on condition that the pixels are included in acolor region extended by the color region extending section,discriminated by the first discrimination unit such that the pixelsbelong to a halftone dot region; and an image process unit that correctsimage data based on a discrimination result obtained by the colorhalftone dot region discrimination unit.
 10. An image processingapparatus according to claim 9, wherein the first discrimination unitcontains: a first sampling unit that samples ahalftone-dot-characteristic-point pixel indicative of halftone dotcharacteristic from image data; a first counter that counts the numberof halftone-dot-characteristic-point pixels in a first region, includinga target pixel, from among the halftone-dot-characteristic-point pixelssampled by the first sampling unit; and a first discriminator thatdiscriminates whether or not a target pixel belongs to a halftone dotregion by comparing a count value of the first counter with a firstthreshold.
 11. An image processing apparatus according to claim 9,wherein the second discrimination unit contains: a second sampling unitthat samples a color pixel from image data; a second counter that countsthe number of color pixels in a second region, including a target pixel,from among the color pixels sampled by the second sampling unit; and asecond discriminator that discriminates whether or not a target pixelbelongs to a color region by comparing a count value of the secondcounter with a second threshold.
 12. An image processing apparatusaccording to claim 9 further including: a third discrimination unit thatdiscriminates whether or not each pixel belongs to an edge region basedon image data; a halftone dot image internal character regiondiscrimination unit that discriminates a pixel belongs to a halftone dotimage internal character region in case the third discrimination unitdiscriminates that the pixel belongs to an edge region as well as thefirst discrimination unit discriminates that the pixel belongs to ahalftone dot region, wherein the image process unit corrects image databased on a discrimination result of the color halftone dot regiondiscrimination unit and a discrimination result of the halftone dotimage internal character region discrimination unit.
 13. An imageprocessing apparatus according to claim 9, wherein, in case at lease apixel that, the second discrimination unit discriminates, belongs to acolor region exists in a third region where a target pixel is centered,the color region extending section regards that the target pixel belongsto a color region regardless of a discrimination result obtained by thesecond discrimination unit and determines that the color region is to beextended.
 14. An image processing method comprising: a step 1 ofdiscriminating whether or not each pixel of image data belongs to ahalftone dot region as well as whether or not each pixel of image databelongs to a color region; a step 2 of extending a color region formedby pixels that, the step 1 has determined, belong to a color region; astep 3 of discriminating pixels that belong to a color halftone dotregion on condition that the pixels are included in a color regionextended by the step 2, and discriminated by the step 1 such that thepixels belong to a halftone dot region; and a step 4 of correcting imagedata based on a discrimination result obtained by the step 3.